1. Field of the Invention
The invention relates to a high voltage inverter device such as a switching regulator, an inverter or the like used in a high voltage power supply unit, a power supply unit for discharge or the like.
2. Description of the Related Art
Atmospheric pressure plasma is applied to various industrial products as a means for surface treatment for improvement of surface quality, removal of contamination or the like. In the case where adhesion, printing, coating or the like is applied to a resin of the like, performing pre-treatment using the atmospheric pressure plasma can improve the wettability thereof.
For example, when trying to coat a printed matter which has been printed with a resin toner by an electrophotographic image forming apparatus with an ultraviolet cure varnish, the varnish on a part printed with the resin toner may be rejected due to a wax component contained in the resin toner. However, performing surface treatment using the atmospheric pressure plasma improves the wettability and therefore enables vanish coating, resulting in improved added value of the printed matter. In order to generate the atmospheric pressure plasma, a high voltage is required and the high voltage is required to be safely obtained by an inverter device.
In an AC inverter device generating a high voltage of several KV or more than 10 KV to 20 KV easily generating the atmospheric pressure plasma, the high voltage within the voltage range can cause electric shock, or ignition, smoking or the like due to spark and is extremely dangerous to the human body. On the other hand, in the safety standard of International Standards IEC60950 (J60950), the input voltage is safe when it is within 60 VDC that is SELV (Safety Extra Low Voltage) or its voltage peak value does not exceed 42.4V. Therefore, it is essential to form a configuration that a voltage within SELV is used as the input voltage of the inverter and a supply power is limited on the input side even if components of the inverter circuit have a dielectric breakdown for any cause.
Hence, as the configuration of the whole power supply unit, there is a unit, which uses the commercial input power supply, brings the output voltage range of its power supply circuit to a voltage within SELV, and generates a high voltage by a high voltage inverter using the voltage as its input.
When the input voltage is within SELV, a boost ratio n that is several tens of times to several hundreds of times the input voltage is required in order to obtain a predetermined output Vout. Here, assuming thatn=Vout/SELV when Vout=15 KV and SELV=48 V, a boost of n=312.5 times is required.
To realize this, there are circuits such as an N-time rectifying circuit and the like such as a transformer, a Cockcroft-Walton circuit and the like. However, the N-time rectifying circuits such as the Cockcroft-Walton circuit and the like perform charge/discharge by a capacitor and can extract an instantaneous single output but have difficulty in continuously extracting output power. Accordingly, there is no choice but to depend on a large-size transformer in order to obtain a stable output.
Comparing this to an article, moving a light article to a high position is comparatively easy, but lifting a heavy article to the high position requires a huge amount of work. Also in the high voltage inverter device, it is necessary to obtain an output power corresponding to a sum of load (weight)×moving distance×height that is not very small several mW but several tens W to several hundreds W.
The general definition to determine the transformer is expressed by the following expressions. Specifically, the number of windings Np of the excitation winding, the current Ip flowing through the excitation winding, and the number of windings Nout of the output winding are obtained by the following expressions.Np=Vin·Ton/Ae·B Ip=Nout·Iout/Np Nout=Vout·Np·Ton/Vin
Here,                Ton: time ratio (sec)        Ae: effective sectional area (cm2) of core        B: magnetic flux density (gauss) Vin: input voltage        Vout: output voltage (V) Iout: output current (A)        
As is appreciated from the definitions, the relation between the magnetic flux density B of a core included in the transformer or the effective sectional area Ae and the number of windings Np of the excitation winding is in inverse proportion, so that there is a limitation. It is necessary that the number of windings Nout of the output winding is a positive integer number that is as small as possible. However, if the number of windings is small, the magnetic flux density B of core is increased to increase the loss, and the core goes into magnetic saturation and loses the function as the transformer. Alternatively, if the number of windings is too large, the winding length is increased, resulting in increased loss due to the current flowing therethrough.
As depicting by a B-H curve of a ferrite core in FIG. 13, the magnetic flux density B of core changes in almost proportional to the intensity H of the magnetic field only in a specific range indicated by ΔB and the core reaches magnetic saturation when the intensity H of the magnetic field exceeds the range indicated by ΔB. Accordingly, the core functions as a transformer only within the range. On the B-H curve depicted in FIG. 13, the area of a portion surrounded by paths when the intensity H of the magnetic field increases and decreases (an oblique line portion) is generally called a hysteresis loss (core loss). For such a reason, the number of windings Np of the excitation winding is only within a specific range, and the output that can be extracted from the transformer depends on the combination of the number of windings Np of the excitation winding and the magnetic flux density B of core but is eventually limited to the specific range.
If a necessary and sufficient magnetic flux B can be extracted, the deterioration in characteristics of the transformer which will be described below is not caused. However, the magnetic flux is actually not sufficient due to the material of core (for example, there are ferrite 0.2 to 0.3 tesla, silicon steel plate 1 tesla: however, depending on the frequency desired to be used, amorphous 1 tesla, permalloy and the like).
Further, the number of windings Np of the excitation winding and the number of windings Nout of the output winding are in a proportional relation. What is a technical problem here is that since generally the sizes of Np and Nout are decided substantially by the output voltage, the number of windings Nout of the output winding necessarily becomes large when the input voltage Vin is low and the boost ratio n is very high, thus causing an increase in inter-winding capacitance and an increase in inter-layer capacitance and so on. This causes the following problems.                An inductance required as a transformer cannot be obtained at an operating frequency desired to be used.        The range of frequency of the transformer is narrow.        The dielectric loss increases.        The loss due to proximity effect caused by a high voltage increases.        
These decrease the performance of the transformer.
Hence, conventional switching converters include one that is described, for example, in Patent Document 1 (JP H10-144544 A), and this switching converter is a separately-excited ON-OFF DC power supply that has a DC input power supply, a primary winding (excitation winding) having divided windings in one transformer, and two output windings on the output side thereof.
Further, a high voltage power supply circuit described in Patent Document 2 (JP 3152298 B) switches the exciting current of two primary windings (excitation windings) of an insulated high-voltage transformer by a pair of switching elements having an ON duty fixed to 50% and operating in a push-pull mode, and rectifies and smoothes the output of one secondary winding (output winding) to obtain a DC high voltage.